Display devices have evolved to process and display increasingly large amounts of information. Flat panel display technologies have been recently conceived and developed for display devices having small thickness, light weight, and low power consumption. Among these technologies, the liquid crystal display (LCD) device is already widely used for notebook computers, desktop monitors, and other application because of its superior resolution, color image display, and image quality.
Of the different types of known liquid crystal displays (LCD) and active matrix LCD (AM-LCD), which have thin film transistors (TFT) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images.
An LCD device includes an upper substrate, a lower substrate, and a liquid crystal layer interposed between the upper and lower substrates. The LCD device uses an optical anisotropy of a liquid crystal material and produces an image by varying the transmittance of light according to the arrangement of liquid crystal molecules by an electric field.
The lower substrate includes thin film transistors and pixel electrodes. The lower substrate is fabricated through repeated photolithography processes to pattern a previously formed thin film. The upper substrate, which is usually referred to as a color filter substrate, includes a color filter layer for displaying color images. The color filter layer commonly includes color filter patterns of red (R), green (G), and blue (B).
FIG. 1 is an exploded perspective view illustrating a liquid crystal display (LCD) device. The LCD device has first and second substrates 12 and 22, which are spaced apart from and facing each other, and also has a liquid crystal layer 30 interposed between the first and second substrates 12 and 22.
At least one gate line 14 and at least one data line 16 are formed on the inner surface of the first substrate 12 (i.e., the side facing the second substrate 22). The gate line 14 and the data line 16 cross each other to define a pixel region P. A thin film transistor T, as a switching element, is formed at the crossing portion of the gate line 14 and the data line 16. A plurality of such thin film transistors is arranged in a matrix form to correspond to other crossing portions of gate and data lines. A pixel electrode 18, which is connected to the thin film transistor T, is formed in the pixel region P. The first substrate 12, which includes the thin film transistors T and the pixel electrodes 18 arranged in the matrix form, may be commonly referred to as an array substrate.
A black matrix 25 is formed on the inner surface of the second substrate 22 (i.e., the side facing the first substrate 12). The black matrix 25 has openings corresponding to respective pixel regions P and has a lattice shape surrounding each pixel region P. The black matrix 25 covers the gate line 14, the data line 16 and the thin film transistor T. A color filter layer 26 is formed in each opening of the black matrix 25 and includes three color filters of red (R) 26a, green (G) 26b, and blue (B) 26c sequentially arranged. Each color filter corresponds to the pixel region P. A common electrode 28 is formed on an entire surface of the second substrate 22 including the black matrix 25 and the color filter layer 26 and is transparent. The second substrate 22, which includes the black matrix 25, the color filter layer 26 and the common electrode 28, may be commonly referred to as a color filter substrate.
Although not shown in the figure, a sealant is formed along a peripheral region between the first and second substrates 12 and 22 to prevent the liquid crystal layer 30 from leaking. In addition, alignment layers are formed on top surfaces of the first and second substrates 12 and 22 adjacent to the liquid crystal layer 30 and control initial arrangement of liquid crystal molecules of the liquid crystal layer 30. A polarizer is disposed on at least one outer surface of the first substrate 12 and the second substrate 22.
Furthermore, a backlight unit is disposed over the outer surface of the first substrate 12 and provides light.
In operation, when a scanning pulse is applied to the thin film transistor T through the gate line 14 and the thin film transistor T turns on, a data signal from the data line 16 is provided to the pixel electrode 18 through the thin film transistor T. Then, the liquid crystal molecules of the liquid crystal layer 30 are driven and arranged by an electric field induced between the pixel electrode 18 and the common electrode 28. Thus, various images are produced according to varying transmittance of the light by the arrangements of the liquid crystal molecules.
FIG. 2 is a cross-sectional view illustrating a pixel region of an array substrate for an LCD device. A gate electrode 42 and a gate line (not shown) are formed on a substrate 40. A gate insulating layer 45 is formed on an entire surface of the substrate 40 including the gate electrode 42. A semiconductor layer 48 is formed on the gate insulating layer 45 over the gate electrode 42. The semiconductor layer 48 includes an active layer 48a and an ohmic contact layer 48b. 
A source electrode 50, a drain electrode 52, and a data line (not shown) are formed on the ohmic contact layer 48b. The data line crosses the gate line to define a pixel region P. The source electrode 50 and the drain electrode 52 are spaced apart from each other over the gate electrode 42.
A passivation layer 55 is formed on an entire surface of the substrate 40 including the source and drain electrodes 50 and 52. The passivation layer 55 has a drain contact hole 57 exposing the drain electrode 52. A pixel electrode 59 is formed on the passivation layer 55 in the pixel region P. The pixel electrode 59 is connected to the drain electrode 52 through the drain contact hole 57.
The array substrate is manufactured through a photolithographic process using a mask, which may be referred to as a mask process.
More particularly, a first metallic material is deposited on the substrate 40 and then patterned through a first mask process to thereby form the gate electrode 40 and the gate line. Next, a first insulating material, intrinsic amorphous silicon (a-Si), and impurity-doped amorphous silicon (n+ a-Si) are sequentially deposited, and the deposited intrinsic amorphous silicon and the deposited impurity-doped amorphous silicon are patterned through a second mask process to thereby form the semiconductor layer 48, which includes the active layer 48a and the ohmic contact layer 48b. The deposited first insulating material functions as the gate insulating layer 45. A second metallic material is deposited and then patterned through a third mask process to thereby form the data line, the source electrode 50 and the drain electrode 52. At this time, the ohmic contact layer 48b between the source and drain electrodes 50 and 52 is removed to thereby expose the active layer 48. The gate electrode 42, the semiconductor layer 48, and the source and drain electrodes 50 and 52 constitute a thin film transistor. The exposed active layer 48 acts as a channel of the thin film transistor. Then, a second insulating material is deposited and then is patterned through a fourth mask process to thereby form the passivation layer 55 having the drain contact hole 57 exposing a part of the drain electrode 52. A transparent conductive material is deposited on the passivation layer 55 and then is patterned through a fifth mask process to thereby form the pixel electrode 59.
Each mask process includes several steps of cleaning, coating a photoresist layer, exposing through a mask, developing the photoresist layer, and etching. To reduce the number of processes, a diffraction exposure method or a halftone exposure method has been proposed and developed, and, recently, a lift-off method has been suggested.
In the lift-off method, a photoresist pattern is formed, and a certain pattern is formed by using the photoresist pattern as an etching mask. Then, a material layer is formed on an entire surface of a substrate including the photoresist pattern, and the photoresist pattern is removed. At this time, a portion of the material layer on the photoresist pattern is also removed, and thus an expected pattern is formed. FIG. 3A and FIG. 3B are cross-sectional views illustrating a lift-off method for forming a pixel electrode according to the related art. In FIG. 3A, a gate electrode 63 is formed on a substrate 61 including a thin film transistor region TrA and a pixel region P. A gate insulating layer 65 is formed on the gate electrode 63. A semiconductor layer 67 including an active layer 67a and an ohmic contact layer 67b is formed on the gate insulating layer 65 over the gate electrode 63. Source and drain electrodes 70 and 72 are formed on the semiconductor layer 67. The gate electrode 63, the semiconductor layer 67, and the source and drain electrodes 70 and 72 constitute a thin film transistor Tr. These elements are formed through the same processes as the above-mentioned array substrate, which may be manufactured through five-mask processes. An inorganic insulating layer is formed on the substrate 61 including the thin film transistor Tr, and a photoresist layer is formed on the inorganic insulating layer.
The photoresist layer is exposed to light through a mask and is developed to thereby form a photoresist pattern 91. The photoresist pattern 91 covers the thin film transistor Tr and exposes a part of the drain electrode 72. Although not shown in the figure, the photoresist pattern 91 also covers a gate line and a data line. The inorganic insulating layer is patterned by using the photoresist pattern 91 as an etching mask to thereby form a passivation layer 77. The passivation layer 77 covers the thin film transistor Tr, the gate line and the data line and exposes the part of the drain electrode 72 and a part of the gate insulating layer 65. The passivation layer 77 is over-etched, and thus there exists an under cut structure that an edge of the passivation layer 77 is disposed inside an edge of the photoresist pattern 91. The exposed gate insulating layer 65 may be removed, and thus the substrate 61 may be exposed.
A transparent conductive material layer 80 is formed on an entire surface of the substrate 61 including the passivation layer 77 and the photoresist pattern 91. The transparent conductive material layer 80 is disconnected around the edges CA of the passivation layer 77 and the photoresist pattern 91 due to the under cut structure. Additionally, a lower surface of the photoresist pattern 91 adjacent to the edge of the passivation layer 77 is exposed, and thus a stripper for removing the photoresist pattern 91 can permeate into an interface between the passivation layer 77 and the photoresist pattern 91 through the exposed lower surface of the photoresist pattern 91.
In FIG. 3B, the substrate 61 including the transparent conductive material layer 80 is exposed to a stripper, and the stripper permeates into the interface between the passivation layer 77 and the photoresist pattern 91 around the under cut structure. The photoresist pattern 91 is removed with a portion of the transparent conductive material layer 80 on the photoresist pattern 91, and the transparent conductive material layer remains in the pixel region P. The remaining transparent conductive material layer functions as a pixel electrode 82.
However, in the lift-off method of the related art, since a portion of the photoresist pattern contacting the stripper, that is, an exposed surface of the photoresist pattern due to the under cut structure, is very small, it takes a long time to remove the photoresist pattern: for example, more than eight minutes. Therefore, productivity is lowered. Meanwhile, if the substrate is exposed to the stripper material for too much time or a temperature of the stripper is high in order to increase a speed of the process, the source and drain electrodes as well as the photoresist pattern may be removed or damaged. Accordingly, the pixel electrode may poorly contact the drain electrode or may be disconnected to the drain electrode.